The objective assessment of the visual field using multi-focal stimulation has been described recently in a number of publications, including: “The topography of visual evoked response properties across the visual field”, Baseler H A et al., Electroencephal. Clin. Neurophysiol. 1994; 90: 65-81; “Multifocal topographic visual evoked potential: improving objective detection of local visual field defects”, Klistorner A I et al., Invest Ophthalmol Vis Sci 1998; 39: 937-950; “Objective VEP perimetry in glaucoma—Asymmetry analysis to identify early deficits”, Graham S L et al., J Glaucoma 2000; 9; 10-19; “Objective perimetry in glaucoma”, Klistorner A and Graham S L, Ophthalmology 2000; 107: 2283-2299; “Multifocal VEP and ganglion cell damage: applications and limitations for the study of glaucoma”, Hood D C et al., Progress in Retinal & Eye Research 2001; 22: 201-251; “A Multifocal objective perimetry in the detection of glaucomatous field loss”, Goldberg I. et al., American Journal of Ophtalmology 2002; 133: 29-39; “Clinical application of the multifocal VEP in glaucoma practice”, Graham, S L et al., Arch Ophthalmol 2005; 123, 729-793.
Using different types of multifocal stimulus presentation, it is possible to perform simultaneous stimulation of a large number of locations of the visual field. Such multifocal stimulus presentations are described, for example, in U.S. Pat. No. 4,846,567 (Sutter), and International (PCI) Application No. PCT/AU00/01483 (Malov, I.). It is possible to record visually evoked cortical potentials (VEP) and electroretinograms (ERG) from all areas of the visual field. For the VEP, various electrode placements have been used.
One representation of the visual field is reported with multichannel bipolar recordings, as described in “Objective perimetry in glaucoma”, Klistorner A. et al., Ophthalmology 2000; 107: 2283-2299, and International (PCT) Application No. PCT/AU99/00340 (Klistorner A et al.) and corresponding U.S. Pat. No. 6,477,407. Good correlation has been described between the multifocal VEP and visual held loss in glaucoma, and with the delay of multifocal VEP latency in optic neuritis in “Multifocal Visual Evoked Potential analysis of inflammatory or demyelinating optic neuritis”, Fraser, C et al., Ophthalmology 2006, 113, 315-323.
The stimulus has traditionally been presented on a cathode ray tube (CRT) screen with high luminance and contrast characteristics to a subject undergoing examination. The use of virtual reality goggles has also been described in International (PCT) Application No. PCT/AU01/00423 (Graham S L et al.). However, not all current screens are suitable for standard rapid pseudorandom sequence presentation, due to slow response times and/or decay rates of those screens. In particular, Liquid Crystal Display (LCD) screens and LCD-based goggles exhibit relatively slow decay rates and are consequently less than optimal for displaying stimuli to patients.
Different modes of stimulation have been described. One of the more commonly used modes of stimulation is rapid reversal driven by m-sequences, as described, for example, in “The topography of visual evoked response properties across the visual field”, Baseler H A et al., Electroencephal. Clin. Neuophysiol. 1994; 90: 65-81, “Multifocal topographic visual evoked potential: improving objective detection of local visual field defects”, Klistorner A I et al., Invest Ophthalmol Vis Sci 1998; 39: 937-950; “Objective VEP perimetry in glaucoma—Asymmetry analysis to identify early deficits”, Graham S L et al., J Glaucoma 2000; 9: 10-19, “Objective perimetry in glaucoma”, Klistorner A et al., Ophthalmology 2000; 107: 2283-2299, “Multifocal VEP and ganglion cell damage: applications and limitations for the study of glaucoma”, Hood D C et al., Progress in Retinal & Eye Research 2001; 22: 201-251. Alternative modes include use of families of sequences as described by Malov (International (PCT) Application No, PCT/AU00/01483) and used clinically in several studies, such as, for example, “Multifocal objective perimetry in the detection of glaucomatous field loss”, Goldberg I et al, American Journal of Ophthalmology 2002; 133: 29-39, “Clinical application of the multifocal VEP in glaucoma practice”, Graham, S L et al., Arch Ophthalmol 2005, 123, 729-793. Other modes of stimulation, such as very slow stimulation rates with pattern pulse stimuli, have also been described, such as, for example, in “Localization of visually evoked cortical activity in humans”, Srebro, R., J Physiol 1985, 360: 233-246, and “The topography of scalp potentials evoked by pattern pulse stimuli”, Srebro, R., Vision Research, 1987, 27: 901-914. This was recently revisited as a technique in “The Pattern-Pulse Multifocal Visual Evoked Potential”, James, A. C., Investigative Opthalmology & Visual Sciences, February 2003, Vol. 44, No. 2.
The derivation of the VEP signal requires multiple repetitions to achieve a good signal to noise ratio (SNR), which in turn determines the total time required to complete a test. The reproducibility of the signal between tests is limited by the SNR of an individual and by differences between test systems.
Thus, a need exists to provide a stimulus protocol that improves VEP signal responses and thus improves SNR with a shortened test time. Further, a need exists to provide a stimulus protocol that is influenced minimally by differences in screen characteristics, such as slow decay rate.